A study of the transport characteristics of the peritubular and luminal borders of the mammalian proximal tubule cell is proposed. The presence of a high conductance extracellular pathway precludes measurement of membrane function in the intact proximal tubule. Therefore, while recent research has provided insight into the nature of the transport of solutes and water across this segment of the renal tubule, little is known about the cellular mechanisms involved. Individual cells will be isolated and aspirated into a small capillary. The cell will separate two fluid compartments. Methods to eliminate or minimize the presence of any high conductance extracellular pathway are proposed. If this can be achieved, then proper orientation of the cell in the capillary would permit the electrical isolation of a fraction of the cell surface area. Microelectrodes will be used to measure the voltage gradient and the electrical conductance across both the luminal and peritubular membranes. The partial ionic conductance of each membrane to various ions will be determined. This information would permit us to determine whether or not the transmembrane potential is a diffusion potential. Finally, methods are proposed to meaure both solute and water movement across a single cell. The techniques proposed may provide the first direct measurements of the transport properties of the proximal tubule cell. These data would improve our understanding of the integration of cellular properties and interspace properties which characterize the "leaky epithelia". Ultimately this methodology may be used to elucidate the sites and mechanism of action of various drugs and hormones.